1. Field of the Invention
This invention relates to a spark plug with a novel central electrode which is used in an internal combustion engine, and the process for production thereof.
2. Description of the Prior Art
The spark portion of the central electrode of a spark plug (e.g., used in an internal combustion engine) is subject to very severe conditions; for example, the spark portion is exposed to the maximum temperature in a combustion chamber, typically nearly 1,000.degree. C. The spark portion, therefore, is required not only to be durable to such high temperatures, but also to have good mechanical durability with respect to spark discharge and good chemical durability with respect to combustion gases.
It has heretofore been known that platinum, gold and like metals have excellent characteristics as a central electrode material, and in some special spark plugs, a noble metal (e.g., platinum, palladium, gold, silver, etc.) wire electrode has been used. These metals, however, are expensive, and, in general, therefore, a heat-resistant alloy made mainly of nickel is more commonly used. When a spark plug obtained using such a nickel alloy is used for a long period of time, the spark portion of the spark plug becomes worn and the spark gap between electrodes is extended. This gives rise to the problem that the voltage at which the spark discharge occurs is increased to higher levels than that which can be produced by an electric source, and thus no discharge occurs. In order to overcome the above disadvantage and to increase the durability of the spark plug, a spark plug has been proposed in which the central electrode is enveloped in an insulator and the tip spark portion is made electrically conductive has been described in U.S. Pat. No. 2,265,352, etc.
This type of spark plug has increased resistance to being worn out by spark discharge, combustion heat and combustion gases since the electrical conductivity-imparting part comprises an alumina material and platinum dispersed therein. However, it has the following disadvantage:
It is generally difficult to produce a dense and uniform composite of high melting point ceramics and a high melting point metal such as platinum, etc. When a mixed powder of alumina and platinum is sintered, even though it might be sintered in appearance, the product obtained may merely be a mixture comprising alumina with platinum particles dispersed therein, as can be see from a cross-sectional microscopic photographic of such a product, as is illustrated in FIG. 3, i.e., a statistical mixture in which two discrete phases are distributed at random and no continuous matrix phase is formed, since alumina and platinum are chemically inert to each other and their mutual wettability is low. Therefore, when such a product (i.e., a statistical mixture of alumina and platinum) is used as a spark portion of the spark plug electrode and repeatedly exposed to spark discharge, mechanically weak links between the alumina and platinum phases are readily broken, resulting in spattering of the platinum. Thus such a product cannot be used as a spark portion for a long period of time.